acpi: fix QEMU crash when started with SLIC table

acpi: fix QEMU crash when started with SLIC table

if QEMU is started with used provided SLIC table blob,

  -acpitable sig=SLIC,oem_id='CRASH ',oem_table_id="ME",oem_rev=00002210,asl_compiler_id="",asl_compiler_rev=00000000,data=/dev/null
it will assert with:

  hw/acpi/aml-build.c:61:build_append_padded_str: assertion failed: (len <= maxlen)

and following backtrace:

  ...
  build_append_padded_str (array=0x555556afe320, str=0x555556afdb2e "CRASH ME", maxlen=0x6, pad=0x20) at hw/acpi/aml-build.c:61
  acpi_table_begin (desc=0x7fffffffd1b0, array=0x555556afe320) at hw/acpi/aml-build.c:1727
  build_fadt (tbl=0x555556afe320, linker=0x555557ca3830, f=0x7fffffffd318, oem_id=0x555556afdb2e "CRASH ME", oem_table_id=0x555556afdb34 "ME") at hw/acpi/aml-build.c:2064
  ...

which happens due to acpi_table_begin() expecting NULL terminated
oem_id and oem_table_id strings, which is normally the case, but
in case of user provided SLIC table, oem_id points to table's blob
directly and as result oem_id became longer than expected.

Fix issue by handling oem_id consistently and make acpi_get_slic_oem()
return NULL terminated strings.

PS:
After [1] refactoring, oem_id semantics became inconsistent, where
NULL terminated string was coming from machine and old way pointer
into byte array coming from -acpitable option. That used to work
since build_header() wasn't expecting NULL terminated string and
blindly copied the 1st 6 bytes only.

However commit [2] broke that by replacing build_header() with
acpi_table_begin(), which was expecting NULL terminated string
and was checking oem_id size.

1) 602b45820 ("acpi: Permit OEM ID and OEM table ID fields to be changed")
2)
Fixes: 4b56e1e4eb08 ("acpi: build_fadt: use acpi_table_begin()/acpi_table_end() instead of build_header()")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/786
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20211227193120.1084176-2-imammedo@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Denis Lisov <dennis.lissov@gmail.com>
Tested-by: Alexander Tsoy <alexander@tsoy.me>
Cc: qemu-stable@nongnu.org
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>